Focal plane arrays (FPAs) which detect infrared radiation are well known in the art and are used by infrared cameras to provide thermal images. For example, infrared radiation passing through an optical path of the infrared camera is received by infrared detectors of the FPA, which provide thermal image data for pixels of a two-dimensional image.
The quality of thermal images provided by FPAs may be degraded due to non-uniform responses among the individual infrared detectors to incident infrared radiation. Factors contributing to the performance degradation may include, for example, variations in the physical characteristics (i.e., dimensions and locations), infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and/or thermal conductivity of the individual infrared detectors. FPA performance may also be degraded by non-uniform out-of-field infrared radiation from surrounding mechanical components. Because the magnitude of the non-uniformity may be large in comparison to the magnitude of the actual response due to the incident infrared radiation, various techniques are typically used to compensate for the non-uniformity and obtain a desirable signal-to-noise ratio.
For example, the FPA may be calibrated over one or more levels of photon flux by inserting a shutter (i.e., an optical obscuration also referred to as a calibration flag) into the optical path of the infrared camera. The temperature of the shutter may be adjusted to emulate a thermal black body detected by the FPA. The FPA takes one or more data frames or snapshots of the shutter to calibrate its response, and the collected data may then be used to calibrate the FPA to provide a more uniform response. The shutter location is often chosen to be as close as possible to the FPA (i.e., between the FPA and the lens) to reduce the shutter size and thus provide a more compact infrared camera.
Although the above-described shutter calibration technique permits calibration of the FPA for the portion of the optical path between the shutter and the FPA, it does not calibrate the FPA to correct for the thermal non-uniformity of the shutter's paddle and the out-of-field infrared radiation caused by other portions of the optical path including, for example, lenses, windows, mounting hardware, or other components of the infrared camera which may be implemented in front of the shutter (i.e., not between the shutter and the FPA). Such non-uniformities may further degrade FPA performance by radiometrically distorting the thermal image data detected by the FPA. Accordingly, there is a need for an improved approach to the calibration of an FPA within an infrared camera.